Computation of continuous cooling transformation diagrams for the heterogeneous nucleation in Sn-5 mass pct Pb droplets
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I. INTRODUCTION
THE solidification of molten droplets has been an important subject of materials research, as it pertains to many industrial processes involving droplet solidification. The evolution of microstructure in solidifying droplets depends on the path along which the solidification of the droplets proceeds.[1–5] The most critical factor that determines the droplet solidification path is the supercooling that precedes nucleation, the start of droplet solidification. Experimental techniques that have been developed to study droplet supercooling include hot-stage droplet dispersion techniques developed by Turnbull and co-workers,[6,7,8] droplet emulsification methods explored by Turnbull[9] and developed by Perepezko and co-workers,[10,11,12] MacIsaac et al.[13] levitation melting techniques,[14,15,16] and drop tower methods.[17,18] However, most of these techniques only apply to stationary droplets, with the exception of drop tower techniques, and as such only generate knowledge specific to the droplets generated by a specific technique. While such knowledge is of scientific importance, it cannot be immediately extended to predict the droplet supercooling and solidification in industrial processes, such as gas atomization, centrifugal atomization, spray forming, and thermal spraying, in which droplets in various sizes solidify while traveling in a gas. Reports on the exact solidification paths of droplets in these industrial processes therefore are scarce. Studying the exact solidification paths of traveling droplets recently became possible with the advent of the uniform-droplet spray (UDS) process in which monosize alloy droplets of desired size are generated by controlled capillary breakup of laminar jets.[19–23] Because of their identical size and motion, the monosize droplets in a UDS all have an essentially identical solidification path. The amounts of supercooling in Sn-5 mass pct Pb droplets generated by the UDS process have been determined by a loss-compensated calorimetric technique[20,21,23] with the aide of a droplet in-flight solidification model[24] develPING WU, Professor, is with the Department of Applied Physics, School of Science, Tianjin University, Tianjin, 300072 P.R. China. TEIICHI ANDO, Associate Professor, is with the Department of Mechanics and Industrial Engineering, Northeastern University, Boston, MA 02115. Contact e-mail: [email protected] Manuscript submitted February 4, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
oped for the UDS process after a model for gas atomization.[25] The results indicated that oxide formation on the droplet surface catalyzes nucleation in Sn-5 mass pct Pb droplets. Dong et al. calculated continuous-cooling-transformation (CCT) diagrams for droplet nucleation based on the supercooling data using a model based on heterogeneous nucleation catalyzed by droplet surface oxidation.[22] This model assumes, however, that the oxide film covers the entire droplet surface in no time and thus fails to address the gradual oxidation of droplet surface during d
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